Surface Treatment and Passivation for High Electron Mobility Transistors

ABSTRACT

A method includes forming a first III-V compound layer over a substrate; forming a second III-V compound layer over the first III-V compound layer, wherein the first and second III-V compound layers include different materials; forming a first crystalline oxide layer over the second III-V compound layer; and forming a first crystalline interfacial layer over the first crystalline oxide layer.

PRIORITY

This is a continuation of U.S. patent application Ser. No. 15/600,165,filed May 19, 2017, which is a divisional of U.S. patent applicationSer. No. 14/879,908, filed Oct. 9, 2015 and issued U.S. Pat. No.9,666,683, herein incorporated by reference in its entirety.

BACKGROUND

In semiconductor technology, Group III-Group V (or III-V) semiconductorcompounds may be used to form various integrated circuit (IC) devices,such as high power field-effect transistors (FETs), high frequencytransistors, or high electron mobility transistors (HEMTs). A HEMT is afield effect transistor having a two dimensional electron gas (2-DEG)layer close to a junction between two materials with different band gaps(i.e., a heterojunction). The 2-DEG layer is used as the transistorchannel instead of a doped region, as is generally the case for metaloxide semiconductor field effect transistors (MOSFETs). Compared withMOSFETs, HEMTs have a number of attractive properties such as highelectron mobility and the ability to transmit signals at highfrequencies.

When manufacturing HEMTs with III-V compounds, it is desirable topassivate the surface of the III-V compounds with a dielectric materiallayer (passivation layer) to prevent the III-V compounds from freelyreacting with ambient air. Current methods of passivizing the surface ofIII-V compounds are not satisfactory in all respects. Sometimes, theinterfacial density of states (Dit) at the interface between thepassivation layer and the III-V compounds are undesirably high, causingdevice performance issues and reliability issues such as drain currentdegradation, large threshold voltage (Vt) fluctuation, and largeoff-current leakage. Accordingly, improvements in this area are needed.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isemphasized that, in accordance with the standard practice in theindustry, various features are not drawn to scale. In fact, thedimensions of the various features may be arbitrarily increased orreduced for clarity of discussion.

FIGS. 1A and 1B are a flow chart of a method of forming a HEMT accordingto various aspects of the present disclosure.

FIGS. 2A, 2B, 2C, 2D, 2E, 2F, 2G, 2H, 2I, 2J, 2K, 2L, 2M, 2N, 2O, 2P,2Q, 2R, 2S, 2T, and 2U are cross-sectional views of a portion of a HEMTin various stages of a manufacturing flow according to the method inFIGS. 1A and 1B, in accordance with an embodiment.

FIGS. 3A, 3B, 3C, 3D, 3E, 3F, and 3G are cross-sectional views of aportion of a HEMT in various stages of a manufacturing flow according tothe method in FIGS. 1A and 1B, in accordance with another embodiment.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

The present disclosure is generally related to semiconductor devices andmethods of forming the same. More particularly, the present disclosureis related to a high electron mobility transistor (HEMT) and methods offorming the same. One object of the present disclosure is to reduce theinterfacial density of states (Dit) between a passivation layer and aIII-V compound layer in the HEMT.

FIGS. 1A and 1B show a flow chart of a method 10 of forming asemiconductor device, according to various aspects of the presentdisclosure. The method 10 is merely an example, and is not intended tolimit the present disclosure beyond what is explicitly recited in theclaims. Additional operations can be provided before, during, and afterthe method 10, and some operations described can be replaced,eliminated, or moved around for additional embodiments of the method. Anembodiment of the method 10 is described in conjunction with FIGS. 2A-2Uwhich are cross-sectional views of a semiconductor device 100 in variousstages of a gate-last manufacturing process, where a gate electrode isformed after the formation of source and drain (S/D) electrodes. Anotherembodiment of the method 10 is described in conjunction with FIGS. 3A-3Gwhich are cross-sectional views of a semiconductor device 200 in variousstages of a gate-first manufacturing process, where a gate electrode isformed before the formation of source and drain (S/D) electrodes.

The semiconductor devices 100 and 200 are provided for illustrationpurposes and do not necessarily limit the embodiments of the presentdisclosure to any number of devices, any number of regions, or anyconfiguration of structures or regions. The semiconductor devices 100and 200 each include a HEMT. Therefore, the semiconductor devices 100and 200 are also referred to as the HEMTs 100 and 200 respectively.

Referring to FIG. 1A, at operation 12, the method 10 epitaxially growsone or more buffer layers over a substrate 102. The one or more bufferlayers may function to reduce lattice mismatch between the substrate 102and an overlying III-V compound layer to be subsequently formed.Referring to FIG. 2A, two buffer layers 104 and 106 are epitaxiallygrown over the substrate 102 in this embodiment.

In various embodiments, the substrate 102 includes a silicon substrate(e.g. a wafer). Alternatively, the substrate 102 may include a siliconcarbide (SiC) substrate or a sapphire substrate. In embodiments, thesubstrate 102 may be a bulk substrate formed of a bulk material, or acomposite substrate including a plurality of layers that are formed ofdifferent materials.

In an embodiment, the buffer layer 104 includes an aluminum nitride(AlN) layer. The buffer layer 104 may have a thickness in a range fromabout 10 nanometers (nm) to about 300 nm. The buffer layer 104 mayinclude a single layer or a plurality of layers, and may be formed bymetal organic vapor phase epitaxy (MOVPE) or other suitable epitaxymethods.

In an embodiment, the buffer layer 106 includes an aluminum galliumnitride (AlGaN) layer. The buffer layer 106 may have a thickness in arange from about 500 nm to about 1000 nm, and may be formed by MOVPE orother suitable epitaxy methods. In some embodiments, the buffer layer106 is formed to have a graded structure, i.e. the relative amount ofthe respective aluminum and/or gallium content change with depth in thelayer throughout a part or the total thickness of the buffer layer 106.The graded structure may provide for better lattice matching between thesubstrate 102 and a subsequently formed overlying III-V compound layer.

At operation 14, the method 10 (FIG. 1A) epitaxially grows a first III-Vcompound layer 108 over the buffer layer 106. Referring to FIG. 2B, inthe present embodiment, the III-V compound layer 108 is a galliumnitride (GaN) layer. The GaN layer 108 may be epitaxially grown byusing, for example, MOVPE, during which a gallium-containing precursorand a nitrogen-containing precursor are used. The gallium-containingprecursor may include trimethylgallium (TMG), triethylgallium (TEG), orother suitable gallium-containing chemicals. The nitrogen-containingprecursor may include ammonia (NH₃), tertiarybutylamine (TBAm), phenylhydrazine, or other suitable nitrogen-containing chemicals. The III-Vcompound layer 108 may have a thickness in a range from about 0.5 micronto about 10 microns. The III-V compound layer 108 may be undoped.Alternatively, it may be unintentionally doped, such as lightly dopedwith n-type dopants due to a precursor used for forming the III-Vcompound layer 108, with no dopant that may cause the III-V compoundlayer 108 to be n-type or p-type intentionally added.

At operation 16, the method 10 (FIG. 1A) epitaxially grows a secondIII-V compound layer 110 over the first III-V compound layer 108.Referring to FIG. 2C, the III-V compound layer 110 has a band gapgreater than the band gap of the III-V compound layer 108. An interface109 is formed between the III-V compound layers 108 and 110. When theHEMT 100 is in operation, a carrier channel 107 is formed and located inthe III-V compound layer 108 near the interface 109. This carrierchannel 107 is known as a two-dimensional electron gas (2-DEG). In thepresent embodiment, the III-V compound layer 110 is an aluminum galliumnitride (AlGaN) layer. For example, the III-V compound layer 110 may bean Al_(x)Ga_(1-x)N layer with 0.05≤x≤1. In embodiments, the AlGaN layer110 may have a thickness ranging from about 3 nm to about 50 nm. TheAlGaN layer 110 may be epitaxially grown by MOVPE or other suitableepitaxy methods. In one example, the AlGaN layer 110 may be grown usingan aluminum-containing precursor, a gallium-containing precursor, and anitrogen-containing precursor. The aluminum-containing precursor mayinclude trimethylaluminum (TMA), triethylaluminum (TEA), or othersuitable aluminum-containing chemicals. The gallium-containing precursorand the nitrogen-containing precursor may be selected from the samecandidate precursors used for forming the GaN layer 108. In anembodiment, the AlGaN layer 110 has a thickness ranging from about 3 nmto about 50 nm.

The III-V compound layer 110 is grown in a process chamber that may besubstantially free of air. However, air, particularly oxygen, may stillexist in the process chamber. Due to the active nature of the III-Vcompound layer 110, a native oxide layer 112 may form at the top surfaceof the III-V compound layer 110, as shown in FIG. 2C. For example, whenthe III-V compound layer 110 is formed of AlGaN, the native oxide layer112 may include gallium oxide (GaO_(x)) or aluminum oxide (AlO_(x)). Thenative oxide layer 112 has an amorphous structure. Therefore, it isundesirable for the performance and reliability of the HEMT 100 eventhough its thickness may be relatively small (e.g., 1˜2 nm).

At operation 18, the method 10 (FIG. 1A) in-situ treats the native oxidelayer 112 with a gas 114. Here, the term “in-situ” means that the nativeoxide layer 112 is treated in the process chamber where the III-Vcompound layer 110 is grown, without breaking vacuum. Referring to FIG.2D, in the present embodiment, the gas 114 includes ammonia (NH₃). Inanother embodiment, the gas 114 includes one of: NH₃, N₂, H₂, or acombination thereof. Additionally or alternatively, the gas 114 mayinclude Cl₂ or BCl₃. The operation 18 effectively crystallizes orrecrystallizes the native oxide layer 112 into a crystalline oxide layer116 (FIG. 2E). To achieve that purpose, the operation 18 is performed ata suitable temperature. If the temperature is too low, recrystallizationof the native oxide layer 112 may not occur. If the temperature is toohigh, the semiconductor device 100 may be damaged. In an embodiment, theoperation 18 is performed at a temperature ranging from about 300Celsius (° C.) to about 900° C. and at a low pressure, such as lowerthan 300 torr. Further, the operation 18 may be performed continuouslyfor 3 minutes or longer, or intermittently for total treatment durationof 3 minutes or longer. In embodiments, the crystalline oxide layer 116may have a thickness of up to 3 nm. In an embodiment where the nativeoxide layer 112 includes gallium oxide (GaO_(x)), the crystalline oxidelayer 116 includes crystalline gallium oxynitride (GaON). In anembodiment where the native oxide layer 112 includes aluminum oxide(AlO_(x)), the crystalline oxide layer 116 includes crystalline aluminumoxynitride (AlON).

At operation 20, the method 10 (FIG. 1A) forms a crystalline interfaciallayer 118 over the crystalline oxide layer 116. Referring to FIG. 2F,the crystalline interfacial layer 118 may have a single crystallinestructure, or may have a polycrystalline structure. Because both theinterfacial layer 118 and the oxide layer 116 have crystallinestructures, the interfacial density of states (Dit) between thecrystalline interfacial layer 118 and the III-V compound layer 110 islow and the resulting HEMT 100 has small threshold voltage (Vt) shiftand high reliability. In an embodiment, the crystalline interfaciallayer 118 includes a semiconductor material such as aluminum nitride(AlN), indium aluminum gallium nitride (In_(x)Al_(y)Ga_(z)N with x+y+z=1and 0≤x, y, z≤1), and boron nitride (BN). In embodiments, thecrystalline interfacial layer 118 may have a thickness ranging fromabout 5 angstrom (Å) to about 100 Å. In an embodiment, the operation 20is performed in-situ with the formation of the III-V compound layer 110and the treatment of the native oxide layer 112, with no vacuum breakbetween the operations 16, 18, and 20. In an alternative embodiment, theoperation 20 is performed ex-situ, i.e., in a process chamber differentfrom where the III-V compound layer 110 is grown and the native oxidelayer 112 is treated. In various embodiments, the crystallineinterfacial layer 118 may be formed using epitaxy, atomic layerdeposition (ALD), physical vapor deposition (PVD), or chemical vapordeposition (CVD).

At operation 22, the method 10 (FIG. 1A) forms a dielectric passivationlayer 120 over the crystalline interfacial layer 118, as shown in FIG.2G. In some embodiments, the dielectric passivation layer 120 has athickness in a range from about 30 nm to about 500 nm. In an embodiment,the dielectric passivation layer 120 includes silicon oxide, siliconnitride, or a combination thereof. The dielectric passivation layer 120may be deposited using CVD, ALD, PVD, or other suitable methods. Forexample, when comprising silicon nitride, the dielectric passivationlayer 120 may be formed by performing a low pressure chemical vapordeposition (LPCVD) using SiH₄ and NH₃ as precursor gases. The dielectricpassivation layer 120 protects the underlying crystalline interfaciallayer 118, the crystalline oxide layer 116, and the III-V compound layer110 from being damaged by subsequent fabrication processes.

At operation 24, the method 10 (FIG. 1B) forms source and drain (S/D)electrodes 126 (FIG. 2L) over the III-V compound layer 110. In anexemplary embodiment, this involves multiple processes includingphotolithography, etching, and deposition processes, which areillustrated in FIGS. 2H-2L. The photolithography process may includeforming a photoresist (or resist) 122 over the dielectric passivationlayer 120 (FIG. 2H), exposing the resist 122 using a pattern thatdefines geometrical shapes for S/D contact holes, performingpost-exposure bake processes, and developing the resist 122 to form amasking element (also referred to as the masking element 122). Themasking element 122, or a derivative thereof, is then used for etchingS/D contact holes (or openings) 124 through the dielectric passivationlayer 120, the crystalline interfacial layer 118, and the crystallineoxide layer 116, thereby exposing portions of the III-V compound layer110 (FIG. 2I). The etching processes may include one or more dry etchingprocesses, wet etching processes, and other suitable etching techniques.The masking element 122 is subsequently removed, and a metal layer 126is deposited over the dielectric passivation layer 120 and filling inthe S/D contact holes 124 (FIG. 2J). The metal layer 126 may include anelemental metal such as titanium (Ti), cobalt (Co), nickel (Ni),tungsten (W), platinum (Pt), tantalum (Ta), palladium (Pd), andmolybdenum (Mo). Alternatively, the metal layer 126 may include ametallic nitride such as titanium nitride (TiN), a metallic orconductive oxide, or a metal alloy such as aluminum copper (AlCu). Themetal layer 126 may be deposited using CVD, PVD, plating, or othermethods. Subsequently, the metal layer 126 is patterned using anotherphotolithography process and another etching process (FIG. 2K). Thephotolithography process forms and defines a masking element 128 (e.g.,a patterned resist) overlying the metal layer 126. Then, the metal layer126 is etched using the masking element 128 as an etch mask. The etchingprocess stops at the dielectric passivation layer 120. The remainingportions of the metal layer 126 form S/D electrodes (or contacts), whichare also referred to as the S/D electrodes 126. The masking element 128is subsequently removed (FIG. 2L). In an embodiment, a thermal annealingprocess is applied to the S/D electrodes 126. The thermal annealingprocess causes the S/D electrodes 126 to react with the III-V compoundlayers 110 and 108 to form an inter-metallic compound (not shown). Theinter-metallic compound connects to the opposite ends of the carrierchannel 107 and provides for more effective electrical connection to thecarrier channel 107.

At operation 26, the method 10 (FIG. 1B) forms a capping layer 128 overthe S/D electrodes 126 and the dielectric passivation layer 120.Referring to FIG. 2M, the capping layer 128 may include an oxide, anitride, or other dielectric materials. For example, the capping layer128 may include silicon nitride, SiNx with 0.5≤x≤1.33, or silicon oxide,SiOx with 0.5≤x≤2.1. In embodiments, the capping layer 128 may include asingle layer or a multi-layer structure. The capping layer 128 may beformed using CVD, PVD, or other deposition techniques.

At operation 28, the method 10 (FIG. 1B) forms an opening 132 throughthe capping layer 128, the dielectric passivation layer 120, thecrystalline interfacial layer 118, and the crystalline oxide layer 116.Referring to FIG. 2N, shown therein is the semiconductor device 100 withthe opening 132. In the present embodiment, the operation 28 includes aphotolithography process and one or more etching processes. Thephotolithography process forms a masking element 130 (e.g., a patternedresist) over the capping layer 128. The one or more etching processesetch the various layers 128, 120, 118, and 116 using the masking element130 as an etch mask. The one or more etching processes may include dryetching, wet etching, or other etching techniques. A portion of theIII-V compound layer 110 is exposed in the opening 132. Because theIII-V compound layer 110 is very active and the etching process chamberis not totally free of oxygen, a native oxide layer 134 is formed overthe portion of the III-V compound layer 110 that is exposed in theopening 132. The native oxide layer 134 generally has an amorphousstructure and is not desirable for the performance of the HEMT 100.

At operation 30, the method 10 (FIG. 1B) treats the native oxide layer134 with a gas 136, thereby converting the native oxide layer 134 to acrystalline oxide layer 138 (FIGS. 2O and 2P). In the presentembodiment, the operation 30 is in-situ performed in the process chamberwhere the opening 132 is formed. Referring to FIG. 2O, the maskingelement 130 of FIG. 2N is removed and the gas 136 is applied to thenative oxide layer 134 in a controlled environment. In an embodiment,the gas 136 includes one of: NH₃, N₂, H₂, or a combination thereof.Additionally or alternatively, the gas 114 may include Cl₂ or BCl₃. Inan embodiment, the gas 136 is applied in an environment that has atemperature ranging from about 300° C. to about 900° C. and at a lowpressure such as lower than 300 torr. Further, the gas 136 may beapplied continuously for 3 minutes or longer, or intermittently fortotal treatment duration of 3 minutes or longer. The operation 30effectively crystallizes or recrystallizes the native oxide layer 134(e.g., GaO_(x) or AlO_(x)) into the crystalline oxide layer 138 (e.g.,GaON or AlON respectively). In embodiments, the gas 136 may be the sameas or different from the gas 114 (FIG. 2D). Furthermore, the crystallineoxide layers 138 and 116 may have the same composition and thickness, ormay have different compositions and/or thicknesses. In an embodiment,the crystalline oxide layer 138 has a thickness of up to 3 nm.

At operation 32, the method 10 (FIG. 1B) forms another interfacial layer140 over the crystalline oxide layer 138, on sidewalls (or interiorsurfaces) of the opening 132, and over the capping layer 128. Referringto FIG. 2Q, the interfacial layer 140 is formed to have a thicknessranging from about 5 Å to about 100 Å in the present embodiment. In anembodiment, the interfacial layer 140 includes aluminum nitride (AlN).In another embodiment, the interfacial layer 140 includes indiumaluminum gallium nitride In_(x)Al_(y)Ga_(z)N with x+y+z=1 and 0≤x, y,z≤1, or boron nitride (BN). The interfacial layer 140 may have the samecomposition and/or thickness as the interfacial layer 118.Alternatively, the interfacial layers 140 and 118 may have differentcompositions and/or thicknesses. In an embodiment, the interfacial layer140 has a crystalline structure in at least the portion of theinterfacial layer 140 over the crystalline oxide layer 138. To furtherthis embodiment, the interfacial density of states (Dit) between theinterfacial layer 140 and the III-V compound layer 110 is advantageouslylow, which improves the threshold voltage (Vt) stability of the HEMT100. In an embodiment, the operation 32 is performed in-situ with thetreatment of the native oxide layer 134, with no vacuum break betweenthe operations 30 and 32. In an alternative embodiment, the operation 32is performed ex-situ, i.e., in a process chamber different from wherethe native oxide layer 134 is treated. In various embodiments, theinterfacial layer 140 may be formed using epitaxy, atomic layerdeposition (ALD), physical vapor deposition (PVD), or chemical vapordeposition (CVD).

At operation 34, the method 10 (FIG. 1B) forms a gate dielectric layer142 over the interfacial layer 140. Referring to FIG. 2R, the gatedielectric layer 142 includes a portion extending into the opening 132and another portion overlying the capping layer 128 and the interfaciallayer 140. In an embodiment, the gate dielectric layer 142 includessilicon nitride, silicon oxide, or aluminum oxide. The gate dielectriclayer 142 may be deposited using CVD, ALD, PVD, or other depositiontechniques. In an embodiment, the gate dielectric layer 142 is formed tohave a thickness ranging from about 50 Å to about 500 Å.

At operation 36, the method 10 (FIG. 1B) forms a gate electrode 144 overa portion of the gate dielectric layer 142, which is illustrated inFIGS. 2S, 2T, and 2U. Referring to FIG. 2S, a metal layer 144 isdeposited over the gate dielectric layer 142 and filling in the opening132 (FIG. 2R). The metal layer 144 may comprise one or more metals suchas tantalum (Ta), titanium (Ti), tungsten (W), aluminum (Al), orcombinations thereof in various embodiments. The metal layer 144 mayalso comprise nitrides (e.g., TiN or TaN) or carbides of the variousmetals. In embodiments, the metal layer 144 may comprise a single layeror multiple layers, such as a tungsten layer on a metal barrier layer.In embodiments, the metal layer 144 may be formed by CVD, PVD, plating,and/or other suitable processes. Referring to FIG. 2T, the metal layer144 is patterned using a process including a photolithography processand one or more etching processes. The photolithography process definesand forms a masking element 146 (e.g., a patterned resist) overlying themetal layer 144, and the one or more etching processes etch the metallayer 144 using the masking element 146 as an etch mask. The remainingportion of the metal layer 144 becomes the gate electrode, also referredto as the gate electrode 144. In the embodiment shown in FIG. 2T, theone or more etching processes also etch the gate dielectric layer 142and the interfacial layer 140 and stop at the capping layer 128. The oneor more etching processes may include dry etching, wet etching, or otheretching techniques. The masking element 146 is subsequently removed(FIG. 2U). Referring to FIG. 2U, a portion of the gate electrode 144extends into the opening 132 (FIG. 2R) and overlays the III-V compoundlayer 110 through a layered structure 148 (enclosed by a dotted box inFIG. 2U). The layered structure 148 includes a portion of the gatedielectric layer 142, a portion of the interfacial layer 140, and thecrystalline oxide layer 138 which is directly underneath the portion ofthe interfacial layer 140. In an embodiment, the portion of theinterfacial layer 140 has a crystalline structure. Another portion ofthe gate electrode 144 overlays the capping layer 128 through anotherportion of the gate dielectric layer 142 and another portion of theinterfacial layer 140 which may or may not have a crystalline structure.

FIGS. 3A through 3G illustrate cross-sectional views of intermediatestages in the formation of the HEMT 200 using a gate-first approach,i.e. a gate electrode of the HEMT 200 is formed before S/D electrodes ofthe HEMT 200 are formed. Many features of the HEMT 200 are similar tothose of the HEMT 100. For the sake of simplicity, some referencenumerals of the HEMT 100 are repeated in the HEMT 200 to indicate sameor similar features. Further, the initial steps of these embodiments areessentially the same as shown in FIGS. 2A through 2G, and hence thedetails of those formation processes are not repeated herein.

Next, as shown in FIG. 3A, a resist 302 is formed over the dielectricpassivation layer 120. In FIG. 3B, the resist 302 is patterned using alithography process to have an opening 304, and then the dielectricpassivation layer 120, the crystalline interfacial layer 118, and thecrystalline oxide layer 116 are etched through the opening 304 to exposea portion of the III-V compound layer 110. A native oxide layer 134 isformed over the portion of the III-V compound layer 110 because theIII-V compound layer 110 is very active and the etching process chamberis not totally free of oxygen. The native oxide layer 134 has anamorphous structure, and is undesirable for the performance of the HEMT200.

Next, referring to FIG. 3C, the native oxide layer 134 is in-situtreated with a gas 136. As a result, the native oxide layer 134 isconverted to a crystalline oxide layer 138 (FIG. 3D). Next, in FIG. 3E,an interfacial layer 140 is deposited over the dielectric passivationlayer 120 and on interior surfaces of the opening 304. In particular, aportion of the interfacial layer 140 is formed over the crystallineoxide layer 138. In an embodiment, this portion of the interfacial layer140 has a crystalline structure. Therefore, the interfacial density ofstates is low between this portion of the interfacial layer 140 and theIII-V compound layer 110. Further, as shown in FIG. 3E, a gatedielectric layer 142 is deposited over the interfacial layer 140.

Next, in FIG. 3F, a gate electrode 144 is formed over a portion of thegate dielectric layer 142 and filling in the opening 304 of FIG. 3E. Inan embodiment, the gate electrode 144 is formed by depositing a metallayer over the gate dielectric layer 142 and patterning the metal layerusing photolithography and etching processes. Next, in FIG. 3G, two S/Delectrodes 126 are formed on opposite sides of the gate electrode 144.In an embodiment, the S/D electrodes 126 are formed by etching S/D holesinto the layers 142, 140, 120, 118, and 116 to expose portions of theIII-V compound layer 110; depositing a metal layer into the S/D holes;and patterning the metal layer. In an embodiment, an inter-metalliccompound (not shown) is formed as a result of reaction between the S/Delectrodes 126 and the III-V compound layers 110 and 108 by a thermalannealing process.

Although not intended to be limiting, one or more embodiments of thepresent disclosure provide many benefits to a high electron mobilitytransistor (HEMT) and a formation process thereof. For example, duringan intermediate stage of the formation of a HEMT, an III-V compoundlayer is epitaxially grown in a process chamber. The process chamber isnot totally free of oxygen. Consequently, a native oxide layer is formedover the surface of the III-V compound layer. The native oxide layer hasan amorphous structure and may degrade the performance of the HEMT.Embodiments of the present disclosure in-situ treat the native oxidelayer with a gas, thereby converting it into a crystalline oxide layer.Embodiments of the present disclosure further form a crystallineinterfacial layer, in-situ or ex-situ, over the crystalline oxide layer,and further form a dielectric passivation layer over the crystallineinterfacial layer. The crystalline interfacial layer and the crystallineoxide layer reduce the interfacial density of states in an interfacebetween the passivation layer and the III-V compound layer, and improvevarious performances of the HEMT.

In some embodiments of the present disclosure, a gate contact hole isetched to expose a portion of an III-V compound layer of a HEMT. Becausethe etching process chamber is not totally free of oxygen, a nativeoxide layer is formed over the surface of the III-V compound layer inthe gate contact hole. Embodiments of the present disclosure treat theoxide layer with a gas, thereby converting it into a crystalline oxidelayer. Embodiments of the present disclosure further form an interfaciallayer, a portion of which is over the crystalline oxide layer; andfurther form a gate dielectric layer over the interfacial layer. Theportion of the interfacial layer and the crystalline oxide layer reducethe interfacial density of states in an interface between the gatedielectric layer and the III-V compound layer, which improves variousperformances of the HEMT.

In one exemplary aspect, the present disclosure is directed to a methodof forming a High Electron Mobility Transistor (HEMT). The methodincludes epitaxially growing a first III-V compound layer, andepitaxially growing a second III-V compound layer over the first III-Vcompound layer, wherein a first native oxide layer is formed on thesecond III-V compound layer. The method further includes in-situtreating the first native oxide layer with a first gas, therebyconverting the first native oxide layer into a first crystalline oxidelayer. The method further includes forming a first crystallineinterfacial layer over the first crystalline oxide layer, and forming adielectric passivation layer over the first crystalline interfaciallayer.

In another exemplary aspect, the present disclosure is directed to amethod of forming a High Electron Mobility Transistor (HEMT). The methodincludes epitaxially growing a first III-V compound layer comprisinggallium nitride, and epitaxially growing a second III-V compound layercomprising aluminum gallium nitride over the first III-V compound layer,wherein a first native oxide layer is formed on the second III-Vcompound layer. The method further includes in-situ treating the firstnative oxide layer with a first gas, thereby converting the first nativeoxide layer into a first crystalline oxide layer. The method furtherincludes forming a first crystalline interfacial layer over the firstcrystalline oxide layer, and forming a passivation layer over the firstcrystalline interfacial layer.

In yet another exemplary aspect, the present disclosure is directed to aHigh Electron Mobility Transistor (HEMT). The HEMT includes a firstIII-V compound layer having a first band gap and a second III-V compoundlayer having a second band gap over the first III-V compound layer,wherein the second band gap is greater than the first band gap. The HEMTfurther includes a first crystalline oxide layer over the second III-Vcompound layer, a first crystalline interfacial layer over the firstcrystalline oxide layer, a dielectric passivation layer over the firstcrystalline interfacial layer, two source and drain (S/D) electrodesover the second III-V compound layer, a gate electrode between the twoS/D electrodes and over a first portion of the second III-V compoundlayer, and a layered structure between the gate electrode and the firstportion of the second III-V compound layer. The layered structureincludes a portion of a second interfacial layer and a portion of a gatedielectric layer over the portion of the second interfacial layer. In anembodiment, the layered structure further includes a second crystallineoxide layer between the first portion of the second III-V compound layerand the portion of the second interfacial layer.

The foregoing outlines features of several embodiments so that those ofordinary skill in the art may better understand the aspects of thepresent disclosure. Those of ordinary skill in the art should appreciatethat they may readily use the present disclosure as a basis fordesigning or modifying other processes and structures for carrying outthe same purposes and/or achieving the same advantages of theembodiments introduced herein. Those of ordinary skill in the art shouldalso realize that such equivalent constructions do not depart from thespirit and scope of the present disclosure, and that they may makevarious changes, substitutions, and alterations herein without departingfrom the spirit and scope of the present disclosure.

What is claimed is:
 1. A method, comprising: forming a first III-Vcompound layer over a substrate; forming a second III-V compound layerover the first III-V compound layer, wherein the first and second III-Vcompound layers include different materials; forming a first crystallineoxide layer over the second III-V compound layer; and forming a firstcrystalline interfacial layer over the first crystalline oxide layer. 2.The method of claim 1, wherein the forming of the second III-V compoundlayer includes generating a native oxide layer over the second III-Vcompound layer, and wherein the forming of the first crystalline oxidelayer includes converting the native oxide layer into the firstcrystalline oxide layer.
 3. The method of claim 2, wherein theconverting of the native oxide layer into the first crystalline oxidelayer includes treating the native oxide layer with a gas.
 4. The methodof claim 3, wherein the gas includes NH₃.
 5. The method of claim 3,wherein the gas includes NH₃, N₂, H₂, or a combination thereof.
 6. Themethod of claim 1, further comprising: forming a gate dielectric layerover the first crystalline interfacial layer; and forming a gateelectrode over the gate dielectric layer.
 7. The method of claim 1,wherein the first crystalline oxide layer includes gallium oxynitride oraluminum oxynitride, and the first crystalline interfacial layerincludes aluminum nitride, indium aluminum gallium nitride, or boronnitride.
 8. The method of claim 1, further comprising: forming adielectric passivation layer directly over the first crystallineinterfacial layer.
 9. The method of claim 8, further comprising: etchingthe dielectric passivation layer, the first crystalline interfaciallayer, and the first crystalline oxide layer to expose a portion of thesecond III-V compound layer; forming a second crystalline oxide layerover the portion of the second III-V compound layer; and forming asecond crystalline interfacial layer over the second crystalline oxidelayer.
 10. The method of claim 9, further comprising: forming a gatedielectric layer over the second crystalline interfacial layer; andforming a gate electrode over the gate dielectric layer.
 11. A method,comprising: forming a first III-V compound layer over a substrate;forming a second III-V compound layer over the first III-V compoundlayer, wherein the second III-V compound layer has a greater band gapthat the first III-V compound layer; forming a crystalline oxide layerover the second III-V compound layer; forming a crystalline interfaciallayer over the crystalline oxide layer; forming a gate dielectric layerover the crystalline interfacial layer; and forming a gate electrodeover the gate dielectric layer.
 12. The method of claim 11, wherein theforming of the crystalline oxide layer includes treating a native oxidelayer on the second III-V compound layer.
 13. The method of claim 12,wherein the treating of the native oxide layer includes applying a gasincluding NH₃, N₂, H₂, or a combination thereof.
 14. The method of claim12, wherein the treating of the native oxide layer is performed at atemperature ranging from 300° C. to 900° C.
 15. The method of claim 12,wherein the treating of the native oxide layer is performed in-situ withthe forming of the second III-V compound layer.
 16. A semiconductordevice, comprising: a first III-V compound layer; a second III-Vcompound layer over the first III-V compound layer, wherein the firstand second III-V compound layers include different materials; acrystalline oxide layer over the second III-V compound layer; and acrystalline interfacial layer over the crystalline oxide layer.
 17. Thesemiconductor device of claim 16, wherein the second III-V compoundlayer includes aluminum gallium nitride, and the crystalline oxide layerincludes crystalline gallium oxynitride or crystalline aluminumoxynitride.
 18. The semiconductor device of claim 17, wherein thecrystalline interfacial layer includes aluminum nitride, indium aluminumgallium nitride, or boron nitride.
 19. The semiconductor device of claim16, wherein the crystalline interfacial layer includes a singlecrystalline structure or a polycrystalline structure.
 20. Thesemiconductor device of claim 16, further comprising a gate dielectriclayer over the crystalline interfacial layer.